Various types of voice alarm and public address systems have a plurality of loud-speaker enclosures which, when driven by an audio amplifier, can provide entertainment such as music or information programs. Such systems are generally arranged so that the loud-speakers can be disconnected from the ordinary entertainment signals and utilized to provide emergency information, such as a fire alarm signal. When used to provide fire alarm or similar important signals, even on a standby basis, it becomes important to effect periodic circuit checks to ensure that the speakers are in suitable operating condition. This is normally done by a "supervisory" circuit of some type which monitors the condition of the speaker. It is desirable to determine whether the speaker coil may have been shorted since the last supervision check, or whether it may have become open-circuited.
Various approaches to the supervisory problem have been attempted. Sometimes high-frequency signals, above the audio-frequency range, have been applied over the speaker system. Although inaudible to human beings, such signals can be disturbing to pets and other animals. Another approach is to utilize a dual voice coil in each speaker, where one of the coils normally drives the speaker diaphragm and the other coil is magnetically coupled to the first coil. In this way the normal driving coil can induce a signal in the second coil, and the induced signal is monitored for supervisory purposes. However this is inefficient from the power dissipation standpoint.
It is therefore a primary object of the present invention to provide a supervisory circuit for monitoring speaker systems which does not disturb pets or other animals, and which is more efficient than know supervision systems.
It is a more specific object of this invention to provide a simple and economical supervisory circuit which is effective to detect and indicate the presence of either an open coil condition or a shorted coil condition within a speaker enclosure.